1. Field of the Invention
The present invention relates to a planar light source and a planar lighting apparatus, and more particularly relates to a planar light source used in backlight light sources for liquid crystal display devices used in channel letters and in image displays such as television sets, word processors, and computers.
2. Background Information
Liquid crystal display devices having a backlight light source are beginning to be used in portable electronic devices and various kinds of image display devices. A liquid crystal display device is usually made up of a light-guide plate, a liquid crystal cell, a prism sheet, a diffuser, a backlight light source, and so forth (see Japanese Laid-Open Patent Application 2003-57622, for example). However, as the display devices used in liquid crystal displays have become larger in size, we are beginning to see the development of liquid crystal displays which do not make use of a light diffuser, which was an essential constituent element of conventional liquid crystal displays. This is because it is difficult to produce a large light diffuser, and large display devices need to be made thinner and more lightweight.
Also, a backlight light source has usually been used as a planar light source in which the core light source utilizes a linear light source comprising a fluorescent lamp. Nevertheless, out of a desire for lower power consumption and longer service life, there is a move toward using backlight light sources comprising light emitting diode s, with which rapid progress has been made in terms of increasing brightness.
Meanwhile, fluorescent lamps and the like have been used not only in display devices, but also in display refrigerators and so forth, which are used to hold bottled water and other products on display in convenience stores and so on. Ironically, because of their poor temperature characteristics at low temperatures, fluorescent lamps have to be heated with a nearby heater despite the fact that they are used in a refrigerator. In view of this, a transition is underway from fluorescent lamps and other such light sources toward light emitting diodes in order to avoid this situation and to reduce power consumption and extend service life.
When a planar light source whose structure includes no light-guide plate is employed in a display device or the like, the action of guiding the light in planar fashion is diminished. Fluorescent lamps and other such linear light sources as the core light source have been used for backlight light sources, and have been obtained as planar, that is, two-dimensional, light sources. Consequently, if the core light source is a linear light source, that is, one-dimensional, all that needs to be done is to raise the dimension by one.
However, when the core light source is a point light source, that is, zero-dimensional, the dimension has to be raised by two, so it is difficult to attain the appropriate brightness, uniform light emission, and so forth. Specifically, when a light emitting diode constitutes a planar light source whose structure includes no light-guide plate, the portions located directly above the light emitting diode become bright spots, and it is difficult to make the light be emitted uniformly throughout the plane. Also, when red, blue, and green light emitting diode chips are separately packaged and then combined to produce white light, color unevenness results.
Also, with a display refrigerator, the whole front is covered with glass, and a light source cannot be installed at this glass portion. Therefore, due to structural limitations, this requires the use of a system in which light emitting diodes are installed vertically at the edges of sliding doors. In this case, emitting the light over a wider range has been attempted by attaching a side emitter lens or a Lambertian lens to every light emitting diode, but it is difficult to prevent the occurrence of bright lines and bright spots due to the difference in optical paths attributable to the difference in the physical distances from the light emitting diode.